1. Field of the Invention
The invention relates to electrically erasable programmable read-only memories known as EEPROMs. More particularly, it pertains to an architecture of an EEPROM that is programmable and erasable by Fowler-Nordheim effect.
The storage cells of EEPROMs are generally formed by a floating-gate storage transistor and a selection transistor enabling the connection of the storage transistor to a word line and a bit line. The EEPROMs use the principle of the non-volatile storage of charges at the floating gate of the floating-gate transistor. Conventionally, the writing (hereinafter designated by the term "programming") is done through the injection by tunnel effect of electrons from the drain or the source into the floating gate (or from the floating gate into the drain or the source), and the erasure is done through injection by tunnel effect of electrons from the floating gate into the drain or the source (or from the drain or the source into the floating gate). There also exist known EEPROMs called flash EEPROMs. In these memories, there is no selection transistor, and the storage transistors are directly connected to the word and bit lines. In this case, a total erasure of the storage cells is performed. This type of memory makes it possible to reduce the amount of space required by the memory while at the same time retaining equivalent storage capacity.
2. Description of the Related Art
The injection of electrons from the drain (or the source) into the control gate of a storage transistor is conventionally done by the ground connection of the drain (or source) of this transistor, the application of a relatively high positive voltage (on the order of +15 volts) to its control gate to create an attracting field and the application of a moderate positive voltage (+6 to +9 volts) to the source (or the drain) to generate hot electrons. When a sufficiently large number of negative charges have collected at the floating gate, the potential of the floating gate reaches the threshold potential of the transistor and prevents the passage of current into a read mode.
The injection of electrons from the floating gate into the drain (or the source) is done by the application of a relatively high positive voltage (+15 volts) to the drain (or the source), the control gate of the storage transistors being ground-connected, and the source (or the drain) being floating. The negative charges are then extracted from the floating gate and directed towards the drain (or the source) by Fowler-Nordheim effect. The thickness of the oxide between the floating gate and the drain (or the source) must be low enough (in the range of 100 Angstroms or less) to enable the injection by Fowler-Nordheim effect.
This type of memory has drawbacks, especially when effecting the ejection of the electrons from the floating gates:
the drain-substrate current (or source-substrate current) during ejection is relatively high and in practice makes it necessary, for large capacity memories, to have available a high positive voltage source that is external to the circuit,
a relatively high reverse voltage is generated between the drains (or the sources) and the substrate during the ejection. In practice this makes it necessary to use drains (or sources) with dual diffusion, thus reducing the density of implementation of the cells,
the application of a relatively high positive voltage during the erasure on the drains (or the sources) increases the probability of the creation of hot holes by avalanche effect on the surface of the drain-substrate junction (or source-substrate) junction, these holes being trapped in the thin oxide located beneath the floating gate.
For more details on these phenomena, reference may be made to the U.S. Pat. No. 5,077,691 by Advance Micro Devices Inc.
In this document, it is proposed to erase the cells by the application of a relatively high negative voltage (-12 to -17 volts) to the control gates, a relatively low positive voltage (+0.5 to +5 volts) being imposed on the sources, with the substrate being connected to the ground and the drains being left in a state of high impedance. Thus, it is possible to induce a Fowler-Nordheim effect while at the same time keeping a reverse source-substrate voltage lower than +5 volts. The leakage current from the source to the substrate is reduced, enabling the memory to be supplied with a single source of external positive voltage (+5 volts). Furthermore, it enables the use of sources comprising a single diffusion, thus limiting the surface area of the cells. Finally, it is possible to eliminate the creation of hot holes. This increases the reliability of the memory.